This invention relates to high performance semiconductor device packaging.
Semiconductor devices increasingly require lower cost packaging with higher thermal and electrical performance. A common package used for high performance devices is the Ball Grid Array (xe2x80x9cBGAxe2x80x9d). The BGA is a surface mount package that can provide higher thermal and electrical performance, and a lower thickness profile and a smaller footprint, as compared to lead frame based surface mount packages. Improvements are sought in the structure and design of the package, to provide increased thermal and electrical performance and to maintain the established footprint and thickness characteristics of standard BGAs.
Ball grid array assembly semiconductor packages and methods for manufacturing them are described, for example, in U.S. Pat. No. 5,397,921, in U.S. Pat. No. 6,020,638 and in U.S. Pat. No. 6,323,065, hereby incorporated by reference.
A BGA package as described in U.S. Pat. No. 5,397,921 includes a heat spreader having a cavity, typically defined by etching, for receiving the semiconductor die; and an interconnect substrate affixed to the heat spreader, and having an aperture arranged over the opening of the die cavity. The interconnect substrate is a xe2x80x9cflex tapexe2x80x9d, which includes a patterned metal routing layer for electrically interconnecting the semiconductor die to electrical connections external to the semiconductor package.
A package as described in U.S. Pat. No. 6,323,065 includes a heat spreader, a ground plane affixed to the heat spreader, and a flex tape interconnect substrate affixed to the ground plane. An aperture in the ground plane defines a path to the surface of the heat spreader. The interconnect substrate, which may have more than one patterned routing layer, is also provided with an aperture, arranged over the ground plane aperture, so that the aperture walls and the heat spreader surface enclosed within them define a cavity for receiving the semiconductor die. The package is constructed by attaching the ground plane to the heat spreader, using a layer of adhesive which is first attached to the ground plane and then to the heat spreader; attaching the flex tape interconnect substrate onto the ground plane, using a layer of adhesive which is first attached to the flex tape interconnect substrate and then to the ground plane; attaching the semiconductor die onto the heat spreader within the ground plane die aperture, using a die attach epoxy; electrically connecting the semiconductor die to bond sites on the ground plane and on the interconnect substrate; filling the cavity with an encapsulation material to protect the die and the wire bonds from the environment and to provide mechanical protection; and positioning solder balls in vias configured for connection to metal routing layers in the interconnect substrate or to the ground plane.
Generally, the package structure according to the invention includes a heat spreader, a ground plane affixed to the heat spreader, and a flex tape interconnect substrate affixed to the ground plane. An aperture in the ground plane reveals a die attach surface on the heat spreader, and an aperture in the flex tape interconnect structure is aligned with the ground plane aperture such that the aligned apertures together with the revealed ground plane surface enclosed by the ground plane aperture define a die cavity.
In one general aspect the aperture in the ground plane is formed so as to be substantially perpendicular to a ground plane surface. According to the invention the heat spreader, the ground plane, and the flex tape interconnect substrate have specified characteristics.
Particularly, in one general aspect of the invention the heat spreader is provided as a metal sheet or strip, usually copper, having a xe2x80x9cvelvetxe2x80x9d type oxide, usually a velvet black copper oxide, on at least the surface of the heat spreader to which the ground plane is to be affixed. According to the invention, a velvet type oxide is preferred because it provides excellent adhesion to die attach epoxies, to encapsulation materials and to adhesives employed in lamination of the ground plane onto the heat spreader.
In another general aspect of the invention the ground plane is provided as a metal sheet or strip, usually copper, having a gray oxide or, more preferably, a velvet type oxide, usually a velvet black copper oxide, on both upper and lower surfaces. Again, according to the invention, a velvet type oxide is preferred because it provides excellent adhesion to adhesives employed in lamination of the ground plane onto the heat spreader, and to encapsulation materials.
In some embodiments the flex tape interconnect structure includes a two-layer tape; in other embodiments the flex tape interconnect tape includes a three-layer tape. In some embodiments the flex tape interconnect structure includes conductive traces formed from one metal layer; in other embodiments the flex tape interconnect structure includes conductive traces formed from at least two metal layers.
In another general aspect the invention features a method for forming a plurality of packages, by providing a metal heat spreader strip, a metal ground plane strip, and a flex tape interconnect structure, in which the metal strips and the tape are dimensioned so that several individual packages are constructed together on the strip and then are singulated to make the several individual packages. The strips and the tape each are provided with an opening for each of the several packages. The ground plane is laminated onto the heat spreader to form a heat spreaderxe2x80x94ground plane assembly. Slots are formed in the laminated heat spreader ground plane assembly, so that each of the four edges of a package is defined by a slot, and so that each package remains connected to the strip by its four corners, referred to as xe2x80x9cearsxe2x80x9d. The tape interconnect substrate is laminated onto the ground plane. The openings in the ground plane and the interconnect substrate are aligned so that the walls of each opening together with the part of the underlying heat spreader that is exposed within them constitutes a die cavity. A die is attached in the cavity. Wire bonds are formed from the die to bond fingers at selected contact points on conductive traces in the interconnect structure. The die and the wire bonds are encapsulated. Solder balls are attached to ball pad sites on conductive traces in the interconnect structure. Then the xe2x80x9cearsxe2x80x9d are severed at the corners of the individual packages to singulate the packages.